Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit only heat and water as by-products.
Fuel cells include three components: a cathode, an anode and an electrolyte which is sandwiched between the cathode and the anode and passes only protons. Each electrode is coated on one side by a catalyst. In operation, the catalyst on the anode splits hydrogen into electrons and protons. The electrons are distributed as electric current from the anode, through a drive motor and then to the cathode, whereas the protons migrate from the anode, through the electrolyte to the cathode. The catalyst on the cathode combines the protons with electrons returning from the drive motor and oxygen from the air to form water. Individual fuel cells can be stacked together in series to generate increasingly larger quantities of electricity.
While they are a promising development in automotive technology, fuel cells are characterized by a high operating temperature which presents a significant design challenge from the standpoint of maintaining the structural and operational integrity of the fuel cell stack. Maintaining the fuel cell stack within the temperature ranges that are required for optimum fuel cell operation depends on a highly-efficient cooling system which is suitable for the purpose.
During startup of a PEM (polymer electrolyte membrane) fuel cell, the faster a fuel cell stack is able to reach operating temperatures, the better the performance of the fuel cell. Due to localized heating of the MEA (membrane electrode assembly) resulting from the electro-chemical reaction of hydrogen and oxygen, adequate removal of heat from the MEA is required. Previous methods of terminating operation of the coolant pump have proven to help heat up the stack at a faster rate; however, because the coolant in the stack is stationary, hot spots tend to occur in the fuel cell stack. Over time, these hot spots turn into pinholes, which ultimately render the stack non-functional.
The design operating temperature for a fuel cell stack is typically in the 65˜80 degrees C. range. During a cold start from a temperature of 5 degrees C., fuel cell stack waste heat is utilized to rapidly bring the temperature of the stack up to its design operating temperature. When the design operating temperature is reached, a coolant pump is started for rejecting waste heat and preventing temperature overshoot.
It is important that the coolant pump not start too early since this will cause the desired operating temperature not to be reached or to be delayed. However, it has been discovered that coolant will circulate even if the coolant pump is not in operation, especially if the stack is started in cold weather. This is due to the difference in density between hot and cold coolant. When coolant is heated in the stack, it rises into the coolant manifold because it is lighter than the relatively cold coolant in the coolant system piping. The colder coolant, in turn, falls back down into the stack by gravity. This rising of the warm coolant and falling of the cold coolant in the system causes a “Ferris wheel” effect in which warm coolant flows freely from the stack to the system piping and cold coolant flows from the system piping into the stack.
Accordingly, a system and method is needed to circulate coolant within a stack during start-up of the fuel cell in order to retain waste heat in the stack and expedite attainment of the stack to operating temperatures. The circulated coolant maintains homogeneity in temperature among all regions of the stack, thus eliminating or reducing the formation of hot spots in the stack.